A sting in the tale of the Great Storm

Michael Fish didn't know he was staring the most malevolent of winds right in the eye, says Roger Highfield

Devastation: Sheffield Park Garden just after the October 2000 Great Storm

By Roger Highfield

12:00AM BST 18 Jun 2003

A potentially devastating weather phenomenon has been documented for the first time by meteorologists. Dubbed the Sting Jet, it is the source of the most damaging winds that scour Britain in winter, uprooting trees, damaging property and taking lives.

The name was inspired by an expression first used by Norwegian meteorologists four decades ago. When describing the source of the strongest winds to batter their country, they talked of the "poisonous tail of the bent-back weather front".

Prof Keith Browning at the University of Reading and Peter Clark and Tim Hewson of the Met Office have found the sting in the tip of this tail and coined the evocative phrase Sting Jet to describe the extraordinary gales that it spawns.

At the start of next month, the public will have a unique opportunity to find out more about the jet at an exhibition of the very best of British science, held in the Royal Society, London.

Recent studies by Dr Robert Muir-Wood of Risk Management Solutions, a company that weighs up weather risks, have concluded that the Sting Jet already causes more than £600 million of damage in Europe in an average year.

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Because global warming is pumping more heat energy into the Earth's atmosphere, the havoc wreaked by the Sting Jet - and the bill for the damage - may rise still further.

What is perhaps most surprising about this find is that, even to the untrained observer, the business end of the Sting Jet is obvious. From the God's eye perspective of a satellite, the devastating winds of the jet are focused at the hook-like tip of the swirl of clouds in a storm system - such as those in the Great Storm of 1987, and a notorious storm that struck on October 30, 2000.

By studying the Great Storm in detail, Prof Browning and his colleagues have concluded that the jet is responsible for the strongest surface winds. This is the "bent-back front" about which the Norwegians fretted decades ago. It took painstaking analysis, however, to uncover the processes that create the extraordinary winds in the bent front "and fit all the bits together", says Prof Browning, who has studied storms for decades.

The realisation that this cloud formation contains the most violent winds has already produced one immediate benefit in terms of what weathermen call "nowcasting".

By providing a nowcast that shows what is going on at a given moment, a timely warning can be given when the signature of a feature such as the Sting Jet appears on a satellite image.

If a Sting Jet is seen over the Atlantic and is heading our way (as Michael Fish witnessed in 1987, without realising it), then weathermen know that high winds are about to wreak havoc when the mainland is stung by the jet.

"You can then forecast more reliably that, in a couple of hours, you will get trouble," says Prof Browning.

The Sting Jet is born in the storms - cyclones - that routinely occur when winds swirl around regions of low pressure, like water around a plug hole. Sting Jets occur in only a small fraction of cyclones, thought to be those that occur when the drop in barometric pressure is spectacular, what American meteorologists call "bombs" that result from "explosive deepening".

There is calm in the eye of the resulting storm, like any other. There are also strong winds swirling around the edge of the storm system, up to 200 miles or so from its heart, like any other. But what is new is the Sting Jet, a violent stream that gradually descends earthwards at the tip of a hook-shaped cloud formation, producing extreme winds that tear around the southern edge of the storm, as close as 100 miles from its centre.

The Sting Jet is born in the upper atmosphere, at an altitude of around three miles, within the clouds formed from the layers of ascending moist air at the heart of the cyclone - the "cloud head". The jet measures only a few tens of miles across and gradually descends with a slope of about one mile in every 50.

With this decline, the jet of air dries out and passes through the ice and snow particles in the cloud head; the ice crystals evaporate and the jet cools, making it denser. Being denser, the jet sinks more quickly, accelerating its flow earthward. Three hours after the jet was born in the higher atmosphere, the descending Sting Jet hits the ground to create 100mph gales.

Effectively, then, the Sting Jet carves out a path and eventually the cloud dissipates altogether at the hooked end of the cloud head. This creates the tell-tale hook-like pattern that can be seen by satellites.

The good news is that computer models already in use by climatologists and forecasters should be able to reveal more about what a Sting Jet is up to, and how it is developing.

Indeed, this is already possible with the Met's latest Unified Computer Model. Introduced last August, the model can simulate the hooked-cloud formation that showed up strongly in the satellite images and indeed the Sting Jet itself.

But there is much work to be done because current computer models are not able to predict the intensity of the Sting Jet with enough precision. Working out the intensity of the Sting Jet is crucial, since the damage caused by 100mph gusts is disproportionately greater than that caused by 90mph gusts and could mean the difference between keeping a roof and losing one, says Prof Browning.

To predict the Sting Jet's wind speeds accurately, and where and when they strike, processes such as turbulence and the evaporation of ice crystals must be accurately simulated within the Met's supercomputers. "We may have to tweak the physics," he says.

But the good news is that "we now know what we have to do, which is also down to using more computer power", adds his Met Office colleague Peter Clark. With heroic number-crunching, meteorologists can carve up the atmosphere into small horizontal slices, a few hundred metres thick, to "see" the narrow Sting Jet as it blasts through the cloud head to cause mayhem below.

Various studies now suggest that severe weather events, such as storms and flooding, will become more common as climate changes as a result of global warming.

The current computer models used to simulate the effects of warming over decades are too crude and coarse to work out whether these narrow blasts of air at the heart of storms will also become more common, says Mr Clark. But if predictions of more powerful and frequent storms prove correct, Sting Jets will have more opportunities to play havoc with our lives.

Visitors to the Royal Society exhibit by Prof Browning and his colleagues can see a model of the Sting Jet and try their hand at forecasting its location.

There are many other exhibits - for instance, on how the inner ear hears, guppy love, orchid conservation, the science of speaking, the Chief Scientist's work on catalysts, the Beagle 2 Mars probe.

The Royal Society's Summer Science Exhibition takes place at the Society, 6 Carlton House Terrace, London SW1 between July 1 and 3, from 10am (11am first day) to 4.30pm. See www.royalsoc.ac.uk for full details.